Problem: Omar is 30 years older than Gabriela. Eighteen years ago, Omar was 4 times as old as Gabriela. How old is Gabriela now?
Explanation: We can use the given information to write down two equations that describe the ages of Omar and Gabriela. Let Omar's current age be $o$ and Gabriela's current age be $g$ The information in the first sentence can be expressed in the following equation: $o = g + 30$ Eighteen years ago, Omar was $o - 18$ years old, and Gabriela was $g - 18$ years old. The information in the second sentence can be expressed in the following equation: $o - 18 = 4(g - 18)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $g$ , it might be easiest to use our first equation for $o$ and substitute it into our second equation. Our first equation is: $o = g + 30$ . Substituting this into our second equation, we get the equation: $(g + 30)$ $-$ $18 = 4(g - 18)$ which combines the information about $g$ from both of our original equations. Simplifying both sides of this equation, we get: $g + 12 = 4 g - 72$ Solving for $g$ , we get: $3 g = 84$ $g = 28$.